In the medical fields of cardiology and electrophysiology, many tools are used to assess the condition and function of a patient's heart, including the observed frequency, polarity and amplitudes of the PQRST complex associated with a heart cycle. Such tools include classic external ECG systems for displaying and recording the characteristic lead ECG signals from skin electrodes placed on the patient's chest and limbs, ambulatory ECG Holter monitors for continuously recording the ECG or segments thereof from a more limited set of skin electrodes for a period of time, and more recently developed completely implantable cardiac monitors or cardiac pacemakers and pacemaker/cardioverter/defibrillators (PCDS) or implantable cardioverter/defibrillators (ICDs) having the capability of recording EGM segments or data derived from atrial and ventricular EGMS (A-EGMs and V-EGMs) for telemetry out to an external programmer for external storage and display.
One of the problems addressed in the design of implantable PCDs or ICDs is the avoidance of unnecessary electrical shocks delivered to a patient's heart in response to rapid heart rates caused by exercise (sinus tachycardia) or by atrial fibrillation. Such rhythms are known collectively as supraventricular tachycardias (SVTs). Studies have shown that SVTs may occur in up to 30% of ICD patients. While ICDs are generally effective at identifying ventricular tachycardia events, the ICD can occasionally deliver a therapy to treat what is detected as being a ventricular tachycardia when in fact the source of the event is related to a supraventricular tachycardia event. Since delivery of the treatment is painful and disconcerting to the patient, deficiencies in distinguishing ventricular tachycardia events from supraventricular tachycardia events tends to be problematic, making the reduction of the incidences of inappropriate treatment highly desirable.
One approach to the problem of distinguishing between normal ORS complexes present during SVTs from those indicative of a VT is to study the morphology of the QRS complex and discriminate normal heart beats from abnormal ones based on the similarity of the signal to a sample waveform recorded from the normal heartbeat, typically referred to as a template. Since a normal QRS complex, or slow rate rhythm, is generally narrower than the QRS complex during VT, or fast rate rhythm, one of the existing methods to discriminate between VT and normal EGM waveforms is based on the properly measured width of the QRS complex. By creating the template based on information sensed from supraventricular rhythm complexes, the ICD is able to compare cardiac complexes sensed during tachycardia episodes against the supraventricular rhythm template. Based on the results of the comparison, the ICD is able to classify the tachycardia episodes as being either a VT complex or a SVT complex, and delivers therapy according to the classification.
In theory, the shape of the QRS complex in the EGM signal during SVT will not change significantly in most patients, because ventricular depolarizations are caused by normal HIS-Purkinje conduction from the atrium to the ventricle. If high ventricular rates are due to a ventricular tachycardia (VT), one can expect a very different morphology of the electrogram (EGM) signal of the ventricular depolarization (QRS complex) because of a different pattern of electrical activity of the heart during VT. However, in certain instances, such as during the electrode/tissue maturation process, or when the patient begins taking new or additional medications, develops a myocardial infarction, or experiences other physiological changes causing the electrical tissue of the patient to change, the morphology of the normal heart rhythm of the patient may change from that originally used as a basis for creating the template. As a result, since deviation from the “normal” heart rhythm of the patient occurs, the template begins to become corrupted, no longer being representative of the patient's current normal heart rhythm and therefore causing the number of inappropriately delivered therapies to increase.
In addition to reducing delivery of inappropriate therapy, another major consideration to be taken into account in the development of the ICD is the limited battery power of the ICD that is available. Since the batteries supplied in the ICD cannot be replaced after initial implantation of the device without surgical procedures, the entire ICD must typically be surgically replaced once the batteries become depleted, making it very desirable to conserve battery power of the ICD. As a result, one of the ways to conserve battery power is to reduce the current drain by reducing the complexity of the signal processing that must be performed by the ICD, limiting the available solutions to reduction of inappropriate therapy delivery. Accordingly, what is needed is a method for reducing the instances of inappropriate therapy delivery that maximizes conservation of the battery power of the device.